The invention relates to a valve comprising a housing, a rotatable body which can turn inside the housing, and a pair of sealing rings which are arranged axially spaced in the housing and rest sealingly against the outer surface of the rotatable body.
A valve of this type, which is designed as a ball valve, is known from published German patent application No. DE 3,205,025. In the housing a ball-shaped rotatable body is situated so as to be rotatable and floating and serves to block or restrict a flowing medium, and a shaft, sealed by the housing and passing through it, is provided for turning. The sealing or seating rings for the rotatable part are comprised of an appropriate sealing material such as polyamide, graphitic carbon, or PTFE (polytetrafluoroethylene), are solid, and have a closed cross-sectional area. At least one of the sealing rings, which will also be referred to hereinafter as seating rings, is associated with a spring arrangement designed to provide a prestress of the seating ring oriented against the spherical outer surface of the rotatable body. Such spring arrangements require an additional manufacturing and assembly cost. Furthermore, difficulties result when corrosive materials are conducted through the pipes in which such a valve is being used. It is known that valves today are increasingly being coated inside, particularly with synthetic resin material, above all in order to satisfy the safety requirements regarding corrosive materials or to avoid the use of expensive corrosion-resistant metals. Plastics like PTFE, PFA (perfluoroalkoxy) and FEP (Fluorinated ethylene propylene) absolutely resist corrosion by practically all acids and alkaline solutions of any concentration and can thus be used anywhere. In such a situation spring parts made from metal would pose an additional risk factor or require additional and expensive measures, if indeed it is possible technologically and economically to realize a non-corrosive embodiment.
A sealing ring made of PTFE for sealing the ball of ball valves is known from published German patent application No. DE 3,706,215. The sealing ring has a angle profile section which is open on the back side and has an outer casing which is essentially coaxial with the longitudinal axis as well as an annular disk-shaped sealing collar against which the ball surface lies. The sealing collar is supported on its back side opposite the ball surface by a support ring with an essentially triangular cross-section, against which the sealing collar can be flexibly bent back. It is not possible to change the bearing surface of the sealing ring on the ball surface as a function of the axial load, and a permanent plastic deformation of the sealing collar can already occur after a short while in use.
Furthermore, a sealing element is known from the German Utility Model No. DE-U 94/13,091, particularly for use in adjusting members for liquid or gaseous media.
Disk springs are integrated into the sealing element, which involves a substantial manufacturing cost.
Furthermore, a valve, particularly a ball valve, is known from U.S. Pat. No. 3,940,107, whose sealing ring exhibits a frame which is oriented essentially radially. On the radially inner end of the frame, a sealing lip is provided which lies against the rotatable ball and which has an extension on the side opposite the ball for providing support in the housing. The frame has a recess in both the direction facing the ball and the direction axially opposite the housing, so that sufficient flexibility is provided for the frame and the sealing lip. A defined axial support of the sealing lip via its extension in the housing is not in itself possible as both the sealing lip and the extension yield outwards or inwards in the radial direction under axial strain.
Finally, a ball valve is known from published German patent application No. DE 4,235,913, whose housing is provided with a lining of synthetic resin. The ball lies directly on the lining, where a resilient insert is provided either in the lining itself or in an annular groove of the housing. The manufacturing cost for such an elastic insert is not insignificant, and relatively narrow manufacturing tolerances must be maintained in order to obtain a sufficient seal. The seal function is negatively affected over time due to wear and plastic deformation.
Seating rings made from PTFE or a similar sealing material exhibit plastic deformation such that due to forces encountered during assembly and additionally due to the differential pressure which has to be supported during normal operation, plastic deformation already occurs, and this plastic deformation does not regain its shape, or only does so slowly, thereby raising a danger of reduced sealing power and consequent leakage. A construction whose assembly requires a relatively high sealing force, which in principle is only needed at higher pressures, requires quite a high torque in order to operate even in a state with no pressure. In order to obtain sufficiently reproducible conditions, the permissible manufacturing tolerances must be kept within a small range, especially since the sealing power is additionally influenced by operating conditions such as pressure, temperature, etc. Particularly in valves lined with plastic, rotatable bodies are provided, especially metal balls or conical plugs which are similarly coated with synthetic resin, or balls which consist of other materials, such as oxide ceramics, which are appropriate for contact with corrosive media.